Mixture of dyes for cyan dye donor for thermal color proofing
This invention relates to use of a mixture of dyes in a cyan dye-donor element for thermal dye transfer imaging which is used to obtain a color proof that accurately represents the hue of a printed color image obtained from a printing press. 
     In order to approximate the appearance of continuous-tone (photographic) images via ink-on-paper printing, the commercial printing industry relies on a process known as halftone printing. In halftone printing, color density gradations are produced by printing patterns of dots or areas of varying sizes, but of the same color density, instead of varying the color density continuously as is done in photographic printing. 
     There is an important commercial need to obtain a color proof image before a printing press run is made. It is desired that the color proof will accurately represent at least the details and color tone scale of the prints obtained on the printing press. In many cases, it is also desirable that the color proof accurately represent the image quality and halftone pattern of the prints obtained on the printing press. In the sequence of operations necessary to produce an ink-printed, full-color picture, a proof is also required to check the accuracy of the color separation data from which the final three or more printing plates or cylinders are made. Traditionally, such color separation proofs have involved silver halide photographic, high-contrast lithographic systems or non-silver halide light-sensitive systems which require many exposure and processing steps before a final, full-color picture is assembled. 
     Colorants that are used in the printing industry are insoluble pigments. By virtue of their pigment character, the spectrophotometric curves of the printing inks are often unusually sharp on either the bathochromic or hypsochromic side. This can cause problems in color proofing systems in which dyes as opposed to pigments are being used. It is very difficult to match the hue of a given ink using a single dye. 
     In DeBoer U.S. patent application Ser. No. 514,643, filed Apr. 25, 1990, a process is described for producing a direct digital, halftone color proof of an original image on a dye-receiving element. The proof can then be used to represent a printed color image obtained from a printing press. The process described therein comprises: 
     a) generating a set of electrical signals which is representative of the shape and color scale of an original image; 
     b) contacting a dye-donor element comprising a support having thereon a dye layer and an infrared-absorbing material with a first dye-receiving element comprising a support having thereon a polymeric, dye image-receiving layer; 
     c) using the signals to imagewise-heat by means of a diode laser the dye-donor element, thereby transferring a dye image to the first dye-receiving element; and 
     d) retransferring the dye image to a second dye image-receiving element which has the same substrate as the printed color image. 
     In the above process, multiple dye-donors are used to obtain a complete range of colors in the proof. For example, for a full-color proof, four colors: cyan, magenta, yellow and black are normally used. 
     By using the above process, the image dye is transferred by heating the dye-donor containing the infrared-absorbing material with the diode laser to volatilize the dye, the diode laser beam being modulated by the set of signals which is representative of the shape and color of the original image, so that the dye is heated to cause volatilization only in those areas in which its presence is required on the dye-receiving layer to reconstruct the original image. 
     Similarly, a thermal transfer proof can be generated by using a thermal head in place of a diode laser as described in U.S. Pat. No. 4,923,846. Commonly available thermal heads are not capable of generating halftone images of adequate resolution but can produce high quality continuous tone proof images which are satisfactory in many instances. U.S. Pat. No. 4,923,846 also disclosed the choice of mixtures of dyes for use in thermal imaging proofing systems. The dyes are selected on the basis of values for hue error and turbidity. The Graphic Arts Technical Foundation Research Report No. 38, &#34;Color Material&#34; (58-(5) 293-301, 1985 gives an account of this method. 
     An alternative and more precise method for color measurement and analysis uses the concept of uniform color space known as CIELAB in which a sample is analyzed mathematically in terms of its spectrophotometric curve, the nature of the illuminant under which it is viewed and the color vision of a standard observer. For a discussion of CIELAB and color measurement, see &#34;Principles of Color Technology&#34;, 2nd Edition, p.25-110, Wiley-Interscience and &#34;Optical Radiation Measurements&#34;, Volume 2, p.33-145, Academic Press. 
     In using CIELAB, colors can be expressed in terms of three parameters: L*, a* and b*, where L* is a lightness function, and a* and b* define a point in color space. Thus, a plot of a* v. b* values for a color sample can be used to accurately show where that sample lies in color space, i.e., what its hue is. This allows different samples to be compared for hue if they have similar density and L* values. 
     In color proofing in the printing industry, it is important to be able to match the proofing ink references provided by the International Prepress Proofing Association. These ink references are density patches made with standard 4-color process inks and are known as SWOP (Specifications Web Offset Publications) Color References. For additional information on color measurement of inks for web offset proofing, see &#34;Advances in Printing Science and Technology&#34;, Proceedings of the 19th International Conference of Printing Research Institutes, Eisenstadt, Austria, June 1987, J. T. Ling and R. Warner, p.55. 
     We have found that an acceptable hue match for a given sample is obtained by a mixture of dyes, if the color coordinates of the sample lie close to the line connecting the coordinates of the individual dyes. Thus, this invention relates to the use of a mixture of cyan dyes for thermal dye transfer imaging to approximate a hue match of the cyan SWOP Color Reference. While the individual dyes by themselves do not match the SWOP Color Reference, the use of a suitable mixture of dyes allows a good color space (i.e., hue) match to be achieved. In addition, the mixture of dyes described in this invention provide a closer hue match to the SWOP standard than the preferred dye mixtures of U.S. Pat. No. 4,923,846. 
     Accordingly, this invention relates to a cyan dye-donor element for thermal dye transfer comprising a support having thereon a dye layer comprising a mixture of cyan dyes dispersed in a polymeric binder, at least one of the cyan dyes having the formula: ##STR3## wherein: R 1  and R 2  each independently represents hydrogen; an alkyl group having from 1 to about 6 carbon atoms; a cycloalkyl group having from about 5 to about 7 carbon atoms; allyl; or such alkyl, cycloalkyl or allyl groups substituted with one or more groups such as alkyl, aryl, alkoxy, aryloxy, amino, halogen, nitro, cyano, thiocyano, hydroxy, acyloxy, acyl, alkoxycarbonyl, aminocarbonyl, alkoxycarbonyloxy, carbamoyloxy, acylamido, ureido, imido, alkylsulfonyl, arylsulfonyl, alkylsulfonamido, arylsulfonamido, alkylthio, arylthio, trifluoromethyl, etc., e.g., methyl, ethyl, propyl, isopropyl, butyl, pentyl, hexyl, methoxyethyl, benzyl, 2-methanesulfonamidoethyl, 2-hydroxyethyl, 2-cyanoethyl, methoxycarbonylmethyl, cyclohexyl, cyclopentyl, phenyl, pyridyl, naphthyl, thienyl, pyrazolyl, p-tolyl, p-chlorophenyl, m-(N-methyl-sulfamoyl)phenylmethyl, methylthio, butylthio, benzylthio, methanesulfonyl, pentanesulfonyl, methoxy, ethoxy, 2-methane-sulfonamidoethyl, 2-hydroxyethyl, 2-cyanoethyl, methoxy-carbonyl-methyl, imidazolyl, naphthyloxy, furyl, p-tolylsulfonyl, p-chlorophenylthio, m-(N-methyl sulfamoyl)phenoxy, ethoxycarbonyl, methoxyethoxycarbonyl, phenoxycarbonyl, acetyl, benzoyl, N,N-dimethylcarbamoyl, dimethylamino, morpholino, anilino, pyrrolidino etc.; 
     or R 1  and R 2  can be joined together to form, along with the nitrogen to which they are attached, a 5- to 7-membered heterocyclic ring such as morpholine or pyrrolidine; 
     or either or both of R 1  and R 2  can be combined with R 3  to form a 5- to 7-membered heterocyclic ring; 
     each R 3  independently represents substituted or unsubstituted alkyl, cycloalkyl or allyl as described above for R 1  and R 2  ; alkoxy, aryloxy, halogen, thiocyano, acylamido, ureido, alkylsulfonamido, arylsulfonamido, alkylthio, arylthio or trifluoromethyl; 
     or any two of R 3  may be combined together to form a 5- or 6-membered carbocyclic or heterocyclic ring; 
     or one or two of R 3  may be combined with either or both of R 1  and R 2  to complete a 5- to 7-membered ring; 
     m is an integer of from 0 to 4; 
     R 4  represents an electron withdrawing group such as cyano, alkoxycarbonyl, aminocarbonyl, alkylsulfonyl, arylsulfonyl, acyl, nitro, etc.; 
     R 5  represents an aryl group having from about 6 to about 10 carbon atoms; a hetaryl group having from about 5 to about 10 atoms; or such aryl or hetaryl groups substituted with one or more groups such as are listed above for R 1  and R 2  ; 
     R 6  and R 7  each independently represents an electron withdrawing group such as those described above for R 4  ; or 
     R 6  and R 7  may be combined to form the residue of an active methylene compound such as a pyrazolin-5-one, a pyrazoline-3,5-dione, a thiohydantoin, a barbituric acid, a rhodanine, a furanone, an indandione, etc., 
     and at least one of the other of the dyes having the formula: ##STR4## wherein: R 1 , R 2 , R 3  and m represent the same as above; 
     X represents hydrogen, halogen or may be combined together with Y to represent the atoms necessary to complete a 6-membered aromatic ring, thus forming a fused bicyclic quinoneimine, such as a naphthoquinoneimine; with the proviso that when X is hydrogen, then J represents NHCOR F , where R F  represents a perfluorinated alkyl or aryl group; and with the further proviso that when X is halogen, then J represents NHCOR 8 , NHCO 2  R 8 , NHCONHR 8  or NHSO 2  R 8  ; and with the further proviso that when X is combined with Y, then J represents CONHR 8 , SO 2  NHR 8 , CN, SO 2  R 8  or SCN, in which case, however, R 8  cannot be hydrogen; 
     R 8  is the same as R 1  and R 5  as described above; and 
     Y is R 1 , R 5 , acylamino or may be combined together with X as described above. 
     In a preferred embodiment for compounds according to formula 1 employed in the invention, R 4 , R 6  and R 7  are cyano. In another preferred embodiment, R 1  is C 2  H 5 , C 2  H 4  OH, or n-C 3  H 7 . In yet another preferred embodiment, R 2  is C 2  H 5  or n-C 3  H 7 . In yet still another preferred embodiment, R 3  is hydrogen, OC 2  H 5 , CH 3  or NHCOCH 3 . In another preferred embodiment, R 5  is p-C 6  H 4  Cl, m-C 6  H 4  NO 2  or C 10  H 7 . 
     Compounds included within the scope of formula I employed in the invention include the following: 
     
         __________________________________________________________________________
 ##STR5##                                                                 
Compound                                                                  
      R.sup.1                                                             
            R.sup.2                                                       
                R.sup.3                                                   
                      R.sup.5                                             
__________________________________________________________________________
 1    C.sub.2 H.sub.5                                                     
            C.sub.2 H.sub.5                                               
                CH.sub.3                                                  
                      C.sub.6 H.sub.5                                     
 2    C.sub.2 H.sub.5                                                     
            C.sub.2 H.sub.5                                               
                H     C.sub.6 H.sub.5                                     
 3    C.sub.2 H.sub.4 OH                                                  
            C.sub.2 H.sub.5                                               
                CH.sub.3                                                  
                      C.sub.6 H.sub.5                                     
 4    C.sub.2 H.sub.5                                                     
            C.sub.2 H.sub.5                                               
                OC.sub.2 H.sub.5                                          
                      C.sub.6 H.sub.5                                     
 5    n-C.sub.3 H.sub.7                                                   
            n-C.sub.3 H.sub.7                                             
                NHCOCH.sub.3                                              
                      C.sub.6 H.sub.5                                     
 6    C.sub.2 H.sub.5                                                     
            C.sub.2 H.sub.5                                               
                CH.sub.3                                                  
                      o-C.sub.6 H.sub.4 OCH.sub.3                         
 7    C.sub.2 H.sub.5                                                     
            C.sub.2 H.sub.5                                               
                CH.sub.3                                                  
                      p-C.sub.6 H.sub.4 OCH.sub.3                         
 8    C.sub.2 H.sub.5                                                     
            C.sub.2 H.sub.5                                               
                CH.sub.3                                                  
                      p-C.sub.6 H.sub.4 Cl                                
 9    C.sub.2 H.sub.5                                                     
            C.sub.2 H.sub.5                                               
                CH.sub.3                                                  
                      m-C.sub.6 H.sub.4 NO.sub.2                          
10    C.sub.2 H.sub.5                                                     
            C.sub.2 H.sub.5                                               
                CH.sub.3                                                  
                       ##STR6##                                           
11    C.sub.2 H.sub.5                                                     
            C.sub.2 H.sub.5                                               
                CH.sub.3                                                  
                       ##STR7##                                           
12    C.sub.2 H.sub.5                                                     
            C.sub.2 H.sub.5                                               
                CH.sub.3                                                  
                       ##STR8##                                           
13    C.sub.2 H.sub.5                                                     
            C.sub.2 H.sub.5                                               
                H                                                         
                       ##STR9##                                           
14    C.sub.2 H.sub.5                                                     
            C.sub.2 H.sub.5                                               
                CH.sub.3                                                  
                      p-C.sub.6 H.sub.4 F                                 
15    C.sub.2 H.sub.5                                                     
            C.sub.2 H.sub.5                                               
                CH.sub.3                                                  
                      o-C.sub.6 H.sub.4 Cl                                
16    C.sub.2 H.sub.5                                                     
            C.sub.2 H.sub.5                                               
                CH.sub.3                                                  
                      p-C.sub.6 H.sub.4 CN                                
17    C.sub.2 H.sub.5                                                     
            C.sub.2 H.sub.5                                               
                CH.sub.3                                                  
                      3,4-C.sub.6 H.sub.3 Cl.sub.2                        
18    C.sub.2 H.sub.5                                                     
            C.sub.2 H.sub.5                                               
                CH.sub.3                                                  
                      2,4-C.sub.6 H.sub.3 Cl.sub.2                        
19    C.sub.2 H.sub.5                                                     
            C.sub.2 H.sub.5                                               
                CH.sub.3                                                  
                       ##STR10##                                          
20    C.sub.2 H.sub.5                                                     
            C.sub.2 H.sub.5                                               
                CH.sub.3                                                  
                       ##STR11##                                          
21    C.sub.2 H.sub.5                                                     
            C.sub.2 H.sub.5                                               
                CH.sub.3                                                  
                       ##STR12##                                          
22    C.sub.2 H.sub.4 OH                                                  
            C.sub.2 H.sub.5                                               
                CH.sub.3                                                  
                      p-C.sub.6 H.sub.4 Cl                                
23    C.sub.2 H.sub.4 NH                                                  
            C.sub.2 H.sub.5                                               
                CH.sub.3                                                  
                      p-C.sub.6 H.sub.4 Cl                                
      SO.sub.2 CH.sub.3                                                   
__________________________________________________________________________
24                                                                        
       ##STR13##                                                          
25                                                                        
       ##STR14##                                                          
26                                                                        
       ##STR15##                                                          
27                                                                        
       ##STR16##                                                          
__________________________________________________________________________
 
    
     The above dyes may be prepared analogously to the method described in Example 1 below. 
     Cyan dyes includes within the scope of the above formula II include the following: 
     
         ______________________________________                                    
 ##STR17##                                                                
Compd. R.sup.3   Y            X   J                                       
______________________________________                                    
A      3-CH.sub.3                                                         
                 C.sub.2 H.sub.5                                          
                              Cl  NHCOCH.sub.2 OCH.sub.3                  
B      3-CH.sub.3                                                         
                 NHCOCH.sub.2 OCH.sub.3                                   
                              H   NHCOC.sub.3 F.sub.7                     
C      H         (CHCH).sub.2   CONHCH.sub.3                              
D      3-CH.sub.3                                                         
                 (CHCH).sub.2   CONHCH.sub.3                              
E      3-OCH.sub.3                                                        
                 C.sub.2 H.sub.5                                          
                              Cl  NHCONHC.sub.2 H.sub.5                   
F      2-OCH.sub.3                                                        
                 CH.sub.3     Cl  NHCOC.sub.6 H.sub.5                     
       5-CH.sub.3                                                         
G      3-OC.sub.2 H.sub.5                                                 
                 C.sub.3 H.sub.7                                          
                              Cl  NHSO.sub.2 C.sub.6 H.sub.5              
H      H         (CHCH).sub.2   CN                                        
I      H         (CHCH).sub.2   SO.sub.2 C.sub.4 H.sub.9 -n               
J      3-CH.sub.3                                                         
                 (CHCH).sub.2   CONHC.sub.2 H.sub.4 Cl                    
K      3-C.sub.2 H.sub.5                                                  
                 (CHCH).sub.2   SO.sub.2 NHCH.sub.3                       
L      3-OC.sub.2 H.sub.5                                                 
                 C.sub.2 H.sub.5                                          
                              H   NHCOC.sub.3 F.sub.7                     
M      2-OCH.sub.3                                                        
                 C.sub.6 H.sub.5                                          
                              H   NHCOC.sub.3 F.sub.7                     
N      3-CH.sub.3                                                         
                 C.sub.2 H.sub.4 OCH.sub.3                                
                              Cl  NHCOC.sub.2 H.sub.5                     
______________________________________                                    
     ##STR18##                                                            
P                                                                         
     ##STR19##                                                            
Q                                                                         
     ##STR20##                                                            
R                                                                         
     ##STR21##                                                            
S                                                                         
     ##STR22##                                                            
______________________________________                                    
 
    
     In a preferred embodiment for compounds according to formula II employed in the invention, R 3  is H, CH 3 , OCH 3 , or OC 2  H 5 . In another preferred embodiment, Y is C 2  H 5  or NHCOCH 2  OCH 3  and X is H or Cl. In yet still another preferred embodiment, Y and X are joined together to form a 6-membered aromatic ring. In still another preferred embodiment, J is NHCOCH 2  OCH 3 , NHCOC 3  H 7 , or CONHCH 3 . 
     The compounds of the formula II above employed in the invention may be prepared by any of the processes disclosed in U.S. Pat. No. 4,695,287 and U.K. Patent 2,161,824, the disclosures of which are hereby incorporated by reference. 
     The use of dye mixtures in the dye-donor of the invention permits a wide selection of hue and color that enables a closer hue match to a variety of printing inks and also permits easy transfer of images one or more times to a receiver if desired. The use of dyes also allows easy modification of image density to any desired level. The dyes of the dye-donor element of the invention may be used at a coverage of from about 0.05 to about 1 g/m 2 . 
     The dyes in the dye-donor of the invention are dispersed in a polymeric binder such as a cellulose derivative, e.g., cellulose acetate hydrogen phthalate, cellulose acetate, cellulose acetate propionate, cellulose acetate butyrate, cellulose triacetate or any of the materials described in U.S. Pat. No. 4,700,207; a polycarbonate; polyvinyl acetate; poly(styrene-co-acrylonitrile); a poly(sulfone) or a poly(phenylene oxide). The binder may be used at a coverage of from about 0.1 to about 5 g/m 2 . 
     The dye layer of the dye-donor element may be coated on the support or printed thereon by a printing technique such as a gravure process. 
     Any material can be used as the support for the dye-donor element of the invention provided it is dimensionally stable and can withstand the heat of the laser or thermal head. Such materials include polyesters such as poly(ethylene terephthalate); polyamides; polycarbonates; cellulose esters such as cellulose acetate; fluorine polymers such as polyvinylidene fluoride or poly(tetrafluoroethylene-co-hexafluoropropylene); polyethers such as polyoxymethylene; polyacetals; polyolefins such as polystyrene, polyethylene, polypropylene or methylpentene polymers; and polyimides such as polyimide-amides and polyether-imides. The support generally has a thickness of from about 5 to about 200 μm. It may also be coated with a subbing layer, if desired, such as those materials described in U.S. Pat. Nos. 4,695,288 or 4,737,486. 
     The reverse side of the dye-donor element may be coated with a slipping layer to prevent the printing head from sticking to the dye-donor element. Such a slipping layer would comprise either a solid or liquid lubricating material or mixtures thereof, with or without a polymeric binder or a surface active agent. Preferred lubricating materials include oils or semi-crystalline organic solids that melt below 100° C. such as poly(vinyl stearate), beeswax, perfluorinated alkyl ester polyethers, poly(caprolactone), silicone oil, poly(tetrafluoroethylene), carbowax, poly(ethylene glycols), or any of those materials disclosed in U.S. Pat. Nos. 4,717,711; 4,717,712; 4,737,485; and 4,738,950. Suitable polymeric binders for the slipping layer include poly(vinyl alcohol-co-butyral), poly(vinyl alcohol-co-acetal), poly(styrene), poly(vinyl acetate), cellulose acetate butyrate, cellulose acetate propionate, cellulose acetate or ethyl cellulose. 
     The amount of the lubricating material to be used in the slipping layer depends largely on the type of lubricating material, but is generally in the range of about 0.001 to about 2 g/m 2 .If a polymeric binder is employed, the lubricating material is present in the range of 0.1 to 50 weight %, preferably 0.5 to 40, of the polymeric binder employed. 
     The dye-receiving element that is used with the dye-donor element of the invention usually comprises a support having thereon a dye image-receiving layer. The support may be a transparent film such as a poly(ether sulfone), a polyimide, a cellulose ester such as cellulose acetate, a poly(vinyl alcohol-co-acetal) or a poly(ethylene terephthalate). The support for the dye-receiving element may also be reflective such as baryta-coated paper, polyethylene-coated paper, an ivory paper, a condenser paper or a synthetic paper such as duPont Tyvek®. Pigmented supports such as white polyester (transparent polyester with white pigment incorporated therein) may also be used. 
     The dye image-receiving layer may comprise, for example, a polycarbonate, a polyurethane, a polyester, polyvinyl chloride, poly(styrene-co-acrylonitrile), poly(caprolactone), a poly(vinyl acetal) such as poly(vinyl alcohol-co-butyral), poly(vinyl alcohol-co-benzal), poly(vinyl alcohol-coacetal) or mixtures thereof. The dye image-receiving layer may be present in any amount which is effective for the intended purpose. In general, good results have been obtained at a concentration of from about 1 to about 5 g/m 2 . 
     As noted above, the dye-donor elements of the invention are used to form a dye transfer image. Such a process comprises imagewise-heating a dye-donor element as described above and transferring a dye image to a dye-receiving element to form the dye transfer image. 
     The dye-donor element of the invention may be used in sheet form or in a continuous roll or ribbon. If a continuous roll or ribbon is employed, it may have only the dyes thereon as described above or may have alternating areas of other different dyes or combinations, such as sublimable cyan and/or yellow and/or black or other dyes. Such dyes are disclosed in U.S. Pat. No. 4,541,830, the disclosure of which is hereby incorporated by reference. Thus, one-, two-, three- or four-color elements (or higher numbers also) are included within the scope of the invention. 
     Thermal printing heads which can be used to transfer dye from the dye-donor elements of the invention are available commercially. There can be employed, for example, a Fujitsu Thermal Head (FTP-040 MCSOO1), a TDK Thermal Head F415 HH7-1089 or a Rohm Thermal Head KE 2008-F3. 
     A laser may also be used to transfer dye from the dye-donor elements of the invention. When a laser is used, it is preferred to use a diode laser since it offers substantial advantages in terms of its small size, low cost, stability, reliability, ruggedness, and ease of modulation. In practice, before any laser can be used to heat a dye-donor element, the element must contain an infrared-absorbing material, such as carbon black, cyanine infrared absorbing dyes as described in DeBoer application Ser. No. 463,095, filed Jan. 10, 1990, or other materials as described in the following U.S. application Ser. Nos.: 366,970, 367,062, 366,967, 366,968, 366,969, 367,064, 367,061, 369,494, 366,952, 369,493, 369,492, and 369,491, the disclosures of which are hereby incorporated by reference. The laser radiation is then absorbed into the dye layer and converted to heat by a molecular process known as internal conversion. Thus, the construction of a useful dye layer will depend not only on the hue, transferability and intensity of the image dyes, but also on the ability of the dye layer to absorb the radiation and convert it to heat. 
     Lasers which can be used to transfer dye from dye-donors employed in the invention are available commercially. There can be employed, for example, Laser Model SDL-2420-H2 from Spectra Diode Labs, or Laser Model SLD 304 V/W from Sony Corp. 
     A thermal printer which uses the laser described above to form an image on a thermal print medium is described and claimed in copending Baek and DeBoer U.S. application Ser. No. 451,656, filed Dec. 18, 1989, the disclosure of which is hereby incorporated by reference. 
     Spacer beads may be employed in a separate layer over the dye layer of the dye-donor in the above-described laser process in order to separate the dye-donor from the dye-receiver during dye transfer, thereby increasing the uniformity and density of the transferred image. That invention is more fully described in U.S. Pat. No. 4,772,582, the disclosure of which is hereby incorporated by reference. Alternatively, the spacer beads may be employed in the receiving layer of the dye-receiver as described in U.S. Pat. No. 4,876,235, the disclosure of which is hereby incorporated by reference. The spacer beads may be coated with a polymeric binder if desired. 
     The use of an intermediate receiver with subsequent retransfer to a second receiving element may also be employed in the invention. A multitude of different substrates can be used to prepare the color proof (the second receiver) which is preferably the same substrate used for the printing press run. Thus, this one intermediate receiver can be optimized for efficient dye uptake without dye-smearing or crystallization. 
     Examples of substrates which may be used for the second receiving element (color proof) include the following: Flo Kote Cove® (S. D. Warren Co.), Champion Textweb® (Champion Paper Co.), Quintessence Gloss® (Potlatch Inc.), Vintage Gloss® (Potlatch Inc.), Khrome Kote® (Champion Paper Co.), Consolith Gloss® (Consolidated Papers Co.), Ad-Proof Paper® (Appleton Papers, Inc.) and Mountie Matte® (Potlatch Inc.). 
     As noted above, after the dye image is obtained on a first dye-receiving element, it is retransferred to a second dye image-receiving element. This can be accomplished, for example, by passing the two receivers between a pair of heated rollers. Other methods of retransferring the dye image could also be used such as using a heated platen, use of pressure and heat, external heating, etc. 
     Also as noted above, in making a color proof, a set of electrical signals is generated which is representative of the shape and color of an original image. This can be done, for example, by scanning an original image, filtering the image to separate it into the desired additive primary colors-red, blue and green, and then converting the light energy into electrical energy. The electrical signals are then modified by computer to form the color separation data which is used to form a halftone color proof. Instead of scanning an original object to obtain the electrical signals, the signals may also be generated by computer. This process is described more fully in Graphic Arts Manual, Janet Field ed., Arno Press, New York 1980 (p. 358ff), the disclosure of which is hereby incorporated by reference. 
     A thermal dye transfer assemblage of the invention comprises 
     a) a dye-donor element as described above, and 
     b) a dye-receiving element as described above, 
     the dye-receiving element being in a superposed relationship with the dye-donor element so that the dye layer of the donor element is in contact with the dye image-receiving layer of the receiving element. 
     The above assemblage comprising these two elements may be preassembled as an integral unit when a monochrome image is to be obtained. This may be done by temporarily adhering the two elements together at their margins. After transfer, the dye-receiving element is then peeled apart to reveal the dye transfer image. 
     When a three-color image is to be obtained, the above assemblage is formed three times using different dye-donor elements. After the first dye is transferred, the elements are peeled apart. A second dye-donor element (or another area of the donor element with a different dye area) is then brought in register with the dye-receiving element and the process repeated. The third color is obtained in the same manner. 
    
    
     The following examples are provided to illustrate the invention. 
     EXAMPLE 1 
     Synthesis of Compound 1 
     A. Synthesis of 2-phenyl-1,1,3-tricyano-propene (intermediate for Compound 1) 
     A mixture of benzoylacetonitrile (9.94 g, 0.0685 mole), malononitrile (11.3 g, 0.17 mole), ammonium acetate (5.4 g, 0.07 mole) and ethanol (100 mL) was heated at reflux for 1.5 hours. After cooling to room temperature, the reaction mixture was diluted with water (50 mL) and concentrated hydrochloric acid (7.5 mL) was added dropwise over 5 minutes. The resulting precipitate was collected by filtration and washed with water and ligroin. The yield was 10.0 g (76%), m.p. 92°-98° C. 
     B. Synthesis of Compound 1: 2-phenyl-1,1,3-tricyano -3-(4-diethylamino-2-methylphenylimino)-propene ##STR23## 
     A mixture of the phenyltricyanopropene above (0.58 g, 0.003 mole) and 2-amino-5-diethylaminotoluene hydrochloride (0.64 g, 0.003 mole) in a solution of methanol (30 mL) and water (10 mL) was treated with concentrated ammonium hydroxide (1.8 mL). To this mixture was slowly added a solution of potassium ferricyanide (4.94 g, 0.015 mole) in water (20 mL), keeping the temperature below 20° C. with external cooling. After stirring for 2 hours, the reaction mixture was diluted with water (100 mL) and the resulting precipitate was collected by filtration and washed well with water. The crude dye was crystallized from methanol to yield 0.85 g (81%) of a dark green powder. The dye had a lambda max of 604 nm with a molar extinction coefficient of 44,200 (in acetone solution). 
     EXAMPLE 2 
     Individual cyan dye-donor elements were prepared by coating on a 100 μm poly(ethylene terephthalate) support: 
     1) a subbing layer of poly(acrylonitrile-co-vinylidene chloride-co-acrylic acid) (0.054 g/m 2 ) (14:79:7 wt. ratio); and 
     2) a dye layer containing a mixture of the dyes identified below and illustrated above, (total coverage 0.27 g/m 2 ) and the cyanine infrared absorbing dye illustrated below (0.054 g/m 2 ) in a cellulose acetate propionate binder (2.5% acetyl, 45% propionyl) (0.27 g/m 2 ) coated from dichloromethane. 
     Comparison dye-donors using the separate cyan dyes of the invention and control dye-donors with dye mixtures as described in U.S. Pat. No. 4,923,849 and identified below, each at 0.27 g/m 2 , were also prepared. 
     Cyanine Infrared Absorbing Dye ##STR24## 
     An intermediate dye-receiving element was prepared by coating on an unsubbed 100 μm thick poly(ethylene terephthalate) support a layer of crosslinked poly(styrene-co-divinylbenzene) beads (14 micron average diameter) (0.11 g/m 2 ), triethanolamine (0.09 g/m 2 ) and DC-510® Silicone Fluid (Dow Corning Company) (0.01 g/m 2 ) in a Butvar® 76 binder, a poly(vinyl alcohol-co-butyral), (Monsanto Company) (4.0 g/m 2 ) from 1,1,2-trichloroethane or dichloromethane. 
     Single color images were printed as described below from dye-donors onto a receiver using a laser imaging device as described in U.S. Pat. No. 4,876,235. The laser imaging device consisted of a single diode laser connected to a lens assembly mounted on a translation stage and focused onto the dye-donor layer. 
     The dye-receiving element was secured to the drum of the diode laser imaging device with the receiving layer facing out. The dye-donor element was secured in face-to-face contact with the receiving element. 
     The diode laser used was a Spectra Diode Labs No. SDL-2430-H2, having an integral, attached optical fiber for the output of the laser beam, with a wavelength of 816 nm and a nominal power output of 250 milliwatts at the end of the optical fiber. The cleaved face of the optical fiber (100 microns core diameter) was imaged onto the plane of the dye-donor with a 0.33 magnification lens assembly mounted on a translation stage giving a nominal spot size of 33 microns and a measured power output at the focal plane of 115 milliwatts. 
     The drum, 312 mm in circumference, was rotated at 550 rpm and the imaging electronics were activated. The translation stage was incrementally advanced across the dye-donor by means of a lead screw turned by a microstepping motor, to give a center-to-center line distance of 14 microns (714 lines per centimeter, or 1800 lines per inch). For a continuous tone stepped image, the current supplied to the laser was modulated from full power to 16% power in 4% increments. 
     After the laser had scanned approximately 12 mm, the laser exposing device was stopped and the intermediate receiver was separated from the dye donor. The intermediate receiver containing the stepped dye image was laminated to Ad-Proof Paper® (Appleton Papers, Inc.) 60 pound stock paper by passage through a pair of rubber rollers heated to 120° C. The polyethylene terephthalate support was then peeled away leaving the dye image and polyvinyl alcohol-co-butyral firmly adhered to the paper. The paper stock was chosen to represent the substrate used for a printed ink image obtained from a printing press. 
     The Status T density of each of the stepped images was read using an X-Rite® 418 Densitometer to find the single step image within 0.05 density unit of the SWOP Color Reference. For the cyan standard, this density was 1.4. 
     The a* and b* values of the selected step image of transferred dye or dye-mixture was compared to that of the SWOP Color Reference by reading on an X-Rite® 918 Colorimeter set for D50 illuminant and a 10 degree observer. The L* reading was checked to see that it did not differ appreciably from the reference. The a* and b* readings were recorded and the distance from the SWOP Color Reference calculated as the square root of the sum of differences squared for a* and b*: ##EQU1## e=experiment (transferred dye) s=SWOP Color Reference 
     The hue angle was also determined as follows: 
     
         Hue angle=arctan b*/a* 
    
     The following results were obtained: 
     
                       TABLE 1                                                     
______________________________________                                    
                          Distance      Delta                             
Dye(s)                    From   Hue    Hue                               
(Wt. Ratio)                                                               
         a*       b*      Ref.   Angle  Angle                             
______________________________________                                    
SWOP Cyan                                                                 
         -41.0    -32.8          219                                      
D        -46.9    -20.1   15     203    -15.5                             
D/8 (80:20)                                                               
         -38.5    -32.5   3      220    +1.5                              
D/9 (68:32)                                                               
         -39.0    -33.3   2      221    +1.8                              
D/10 (76:24)                                                              
         -37.2    -30.5   4      219    +0.6                              
8        -25.7    -45.5   20     241    +21.8                             
9        -29.0    -45.8   18     238    +19                               
10       -25.3    -41.7   18     239    +20.1                             
C        -41      -28.7   4      215    -3.7                              
C/8 (88:12)                                                               
         -37.3    -31.6   4      220    +1.6                              
A        -48.1    -19.6   15     202    -16.5                             
A/8 (68:32)                                                               
         -39.8    -32     1      219    0                                 
B        -41.8    -28.7   4      215    -4.2                              
B/8 (80:20)                                                               
         -37.0    -32.9   4      222    +2.9                              
C-1**    -29.9    -38.9   13     233    +13.8                             
C-2***   -28.5    -41.0   15     235    +16.5                             
______________________________________                                    
 **Consisted of a mixture of Control 1, Solvent Blue 63 (analog) and      
 Control 2, Foron Brilliant Blue SR in a 5:20 ratio, of U.S. Pat. No.     
 4,923,846, Table C2 (Example C2).                                        
 ***Consisted of a mixture of Control 3, Cyan dye #3 and Control 2, Foron 
 Brilliant Blue SR in a 7.5:17 ratio, of U.S. Pat. No. 4,923,846, Table C5
 (Example C5).                                                            
 Control 1                                                                
 ##STR25##                                                                
 Control 2                                                                
 ##STR26##                                                                
 Control 3                                                                
 ##STR27##                                                                
 
    
     The above results indicate that by using a mixture of the dyes according t the invention in an appropriate ratio, a hue closely corresponding to that of the cyan SWOP Color Reference was obtained, in comparison to the individual cyan dye images which were much further away from the SWOP Color Reference. Although dye C is apparently close to the aim SWOP hue, the difference in b* is such that the hue angle is smaller and the hue appears greener than the standard. The C/8 mixture is actually closer in hue to the standard because the difference in hue angle is less. The same is true for mixture B/8 in comparison to B alone. 
     The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.